Instruments and gauges used for measuring temperature or pressure are generally enclosed and protected by a lens over the face of the instrument to be protected. Various instruments of this type are disclosed, for example, in U.S. Pat. Nos. 3,630,089; 3,622,038; 3,803,918 and 3,922,919. For certain applications regulation standards, such as U.L. Laboratories, require that the instrument be watertight in order to prevent moisture penetration that could adversely affect instrument performance. The standard imposed by UL requires that the gauge withstand water penetration after a two hour immersion at a one-foot depth. The gauge must also withstand repetitive water sprays following 720 hours of ultraviolet exposure.
Typically, an instrument such as a pressure gauge as disclosed, for example, in U.S. Pat. No. 3,630,089, includes an open end crimpable case, a dial, a protective transparent lens overlying the dial and a resilient gasket compressed intervening between the underside periphery of the lens and dial. Before crimping of the case, a peripheral sidewall on the open end of the case coaxially extends from the case body to receive the dial, lens and intervening gasket. The sidewall is comprised of a corrosion proof ductile material, such as stainless steel. With the elements in place, the open end of the case is compressively crimped tautly about the periphery of the lens to secure the lens against the gasket and the gasket against the dial in place within the case.
The casing structure of U.S. Pat. No. 3,630,089 has been almost exclusively utilized as a pressure gauge on fire extinguishers and has enjoyed many years of commercial success. However, such extinguishers are commonly placed on boats, oil platforms, etc. where high levels of U.V. exposure and water are typically encountered. The casing assembly thereof affords a waterproof and dust-proof enclosure formed of a cup-shaped shell-like corrosion resistant case that is compressively crimped about the peripheral edge of the lens to force and secure the lens against the underlying gasket.
It should be evident that such gauges are mass produced in a highly cost competitive market. As a consequence, each manufacturer strives to reduce fabrication cost of the gauge to the maximum extent possible in order to enhance its competitive market position. Despite recognition of the competitive cost factors, previous efforts to reduce case costs while maintaining U.L. standards have heretofore been largely unsuccessful.